Mixing patterns

The series of synthetically generated mixtures of two water types given in Table 6.4 is drawn in Fig. 6.3 in a fingerprint diagram on semilogarithmic 

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The patterns in a fingerprint diagram provide the same information  

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The equiHbrium approach should not be used for species that are highly sensitive to variations in residence time, oxidant concentration, or temperature, or for species which clearly do not reach equiHbrium. There are at least three classes of compounds that cannot be estimated weU by assuming equiHbrium CO, products of incomplete combustion (PlCs), and NO. Under most incineration conditions, chemical equiHbrium results in virtually no CO or PlCs, as required by regulations. Thus success depends on achieving a nearly complete approach to equiHbrium. Calculations depend on detailed knowledge of the reaction network, its kinetics, the mixing patterns, and the temperature, oxidant, and velocity profiles. 

Real reactors may conform to some sort of ideal mixing patterns, or their performance may be simulated by combinations of ideal models. The commonest ideal models are the following ... 

Grit Chambers Industries with sand or hard, inert particles in their wastewaters have found aerated grit chambers useful for the rapid separation of these inert particles. Aerated grit chambers are relatively small, with total volume based on 3-min retention at maximum flow. Diffused air is normally used to create the mixing pattern shown in Fig. 25-44, with the heavy, inert particles removed by centrifugal action and friction against the tank walls. The air flow rate is adjusted for the specific particles to be removed. Floatable solids are removed in the aerated grit chamber. It is important to provide for... 

Composition changes Temperature changes Mixing patterns Mass transfer Heat transfer... 

Information on the composition and temperature changes is obtained from the rate equation, while the mixing patterns are related to the intensity of mixing and reactor geometry. Heat transfer is referred to as the exothermic or endothermic nature of the reactions and the mass transfer to the heterogeneous systems. 

Meal reactors Refers to tlie assumed mixing patterns in the reactor. 

E(t) has the unit of inverse time (e.g., min ), and in probability terms, E(t) is a density rather than a distribution. It is a fundamental indicator of the flow and mixing pattern in a chemical reactor and its... 

There is approximately a 22% deviation between the experimental and the distribution mean residenee time. However, the main purpose was to use the information from the RTD eurve to improve the reaetor operation. The results of the RTD provided vital information eoneern-ing the effeets of operating eonditions and struetural designs on solid-mixing patterns in fluidized systems. The perfeet mixing funetion was generated by e , where 6 = t/f. Figure 8-19 shows plots of these funetions against dimensionless residenee time 6. 

Gas mixing patterns Volatile products Growth inhibitors... 

Initial comparison of CFSTR runs with similar feed conditions indicates conditions for which the monomer conversion may be dependent on mixing speed. However, when the effects of experimental error in monomer conversion and differences in reaction temperature are considered, the monomer conversion is seen to be relatively independent of mixing speed for rpm equal to or greater than 500. Comparing Run 14 with Run 12 reveals a small decrease in monomer conversion in spite of a rise in reactor temperature of 2°C. This indicated the presence of a small amount of bypassing or dead volume at the lower mixing speed. This imperfect mixing pattern would also be present in Run 15. 

The basic liquid mixing pattern is assumed to remain constant, as shown in Fig. 5.124. 

Figure 5.124. Model for the liquid mixing pattern in a stirred tank.

Figure 5.125. Gas phase mixing pattern and mass transfer for full bubble column.

When dealing with multiple reactions, selectivity or reactor yield is maximized for the chosen conversion. The choice of mixing pattern in the reactor and feed addition policy should be chosen to this end. 

However, it should be noted that there are many practical issues that need to be considered when choosing mixing equipment and mixing patterns, in addition to those for maximizing yield, selectivity or conversion4. This is especially the case when dealing with multiphase reactions4. 

One discouraging problem is the decrease in reactor or combustor performance when a pilot plant is scaled up to a larger commercial plant. These problems can be related to poor gas flow patterns, undesirable solid mixing patterns and physical operating problems (Matsen, 1985). In the synthol CFB reactors constructed in South Africa, first scale-up from the pilot plant increased the gas throughput by a factor of 500. Shingles and McDonald (1988) describe the severe problems initially encountered and their resolution. 

VanDeemter, J. J., Mixing Patterns in Large-scale Fluidized BcdsC Fluidization, (J. R. Grace, and J. M. Matsen, eds.), Plenum Press, New York (1980)... 

As long as the interest is in fields of averaged velocity components and in overall mixing patterns, RANS-based simulations may suffice. Examples of such satisfactory simulation results are plentiful, e.g., Marshall and Bakker (2004) and Montante et al. (2006). When, however, the interest is in the details of the turbulent-flow field and in processes affected by these details, LES is the option to be preferred. From the findings reproduced above and from the validation studies of Derksen and Van den Akker (1999) and Derksen (2001) the general conclusion is that, as long as the spatial resolution is sufficient, LB LES deliver results in excellent agreement with experimental turbulence data. 

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